The invention relates to a process for the preparation of N-alkylpolyhydroxyalkylamines from monoalkylamine and reducing sugar, in which the alkylamine and the reducing sugar are first mixed together in solution, the solution obtained is hydrogenated with hydrogen in the presence of a hydrogenation catalyst and the N-alkylpolyhydroxyalkylamine formed is obtained by removal of the hydrogenation catalyst. Such a process is disclosed, for example, in U.S. Pat. No. 2,016,962, WO-A-92/06073, WO-A-92/08687, WO-A-93/03004 and EP-A-0 536 939, wherein in the three WO publications a detailed description concerning the bringing into contact of monoalkylamine and sugar is given. It is essentially carried out by bringing the sugar and excess alkylamine together at a temperature of approximately 10 to 60xc2x0 C. and atmospheric pressure. Thus in Example I of WO 93/03004 an aqueous methylamine solution, glucose and ethanol are mixed together at room temperature and the solution obtained is allowed to stand overnight. In Examples IX, XI and XII, an aqueous glucose solution is slowly added to an aqueous methylamine solution at a temperature of 10 to 20xc2x0 C. and, in turn, atmospheric pressure, whereupon the solution in Example XI is stirred for approximately 30 minutes and in Example XII for approximately 2 hours.
As emerges from the prior art, in particular WO-A 93/03004, the preparation of the solution or mixture from the sugar and the alkylamine, which is then hydrogenated, has an important role with respect to yield of N-alkylpolyhydroxyalkylamine and its content of coloring by-products and other impurities.
It has surprisingly been found that a high yield of N-alkylpolyhydroxyalkylamine having high color quality and purity is obtained if the solution to be hydrogenated is prepared by mixing the N-alkylamine and the sugar in each case as a solution using pressure and turbulent flow for at most 5 minutes. It is furthermore advantageous if the solution is exposed to the hydrogenation conditions immediately after expiry of the mentioned mixing time.
The process according to the invention accordingly comprises carrying out the mixing together of monoalkylamine and sugar by injecting the monoalkylamine and the reducing sugar, in each case in the form of an aqueous, alcoholic or aqueous/alcoholic solution, simultaneously into a mixing unit and keeping the solution in the mixing unit under turbulence at a temperature of 25 to 60xc2x0 C., preferably 40 to 50xc2x0 C., and a pressure of 50 to 90 bar, preferably 60 to 80 bar, for 3 seconds to 5 minutes, preferably 10 seconds to 1 minute.
The alkylamine is preferably employed as a 10 to 50% strength by weight solution and the sugar as a 40 to 75% strength by weight solution in water, alcohol or in a mixture of water and alcohol, water on its own being preferred as a solvent. The alkylamine solution is preferably fed into the mixing space at a temperature of 10 to 30xc2x0 C. and the sugar solution at a temperature of 40 to 60xc2x0 C.
During the mixing of the two solutions, heat is liberated (exothermic reaction). The specified temperature of the solutions which are introduced into the mixing space is adjusted to the temperature to be adhered to in the mixing of 25 to 60xc2x0 C., preferably 40 to 50xc2x0 C., taking into account the exothermic reaction, so that the mixing temperature mentioned is guaranteed without further measures (such as cooling or heating). In the case of alcohol as a solvent, the C1-C4-alkanols, such as methanol, ethanol and isopropanol and the low molecular weight glycols such as monoethylene glycol, diethylene glycol and propylene glycol or mixtures of these alcohols and glycols are preferred.
The mixing of the alkylamine solution and the sugar solution is carried out according to the invention in a turbulent flow adhering to the specified temperature and pressure values and the specified time (residence time), during which mixing should be carried out under turbulence. The mixing space or mixing unit used is preferably a tube which withstands the pressure to be adhered to. Turbulence can be achieved, for example, by putting the known turbulence-producing internals in the pressure-resistant tube. Such tubes are, as is known, designated as static mixers. To adhere to the residence time according to the invention of 6 seconds to 5 minutes, preferably 10 seconds to 1 minute, the diameter of the tube is clearly to be adjusted to the amount of solution employed. The alkylamine solution and the sugar solution are simultaneously introduced (preferably separately from one another) into the mixing unit, it clearly being possible to use a somewhat higher pressure in comparison with the pressure in the mixing unit.
The solution resulting after mixing in the mixing space is preferably immediately subjected to hydrogenation. In order to achieve immediately subsequent hydrogenation, it is preferred to inject the solution, after the specified residence time in the mixing space, directly into a stirring autoclave into which the hydrogenation catalyst has already been introduced and in which the hydrogenation temperature and the hydrogen pressure necessary for the hydrogenation have already been adjusted. This hydrogen pressure is 25 to 100 bar, preferably 40 to 80 bar, and the hydrogenation temperature is 40 to 85xc2x0 C., preferably 50 to 80xc2x0 C. These hydrogenation conditions are maintained according to the invention until virtually no more hydrogen is absorbed (the hydrogenation time is in general 30 minutes to 5 hours).
According to a preferred embodiment of the process according to the invention, the hydrogenated solution is first thermally formed, stabilized and only then is the N-alkylpolyhydroxyalkylamine removed. The thermal stabilization is preferably itself carried out following the hydrogenation in the stirring autoclave, the hydrogenated solution being heated (at the mentioned temperature of 40 to 100xc2x0 C., preferably 25 to 85xc2x0 C., and under a hydrogen pressure of 50 to 80 bar, preferably 40 to 80 bar) to 95 to 100xc2x0 C., preferably 95 to 105xc2x0 C. The increase in the temperature to these higher values while retaining the hydrogen present is preferably carried out continuously and slowly, that is in a time of approximately 30 to 150 minutes, preferably 40 to 110 minutes (it goes without saying that with the temperature increase the hydrogen pressure in the autoclave also increases to a greater or lesser extent). As soon as the solution has reached the higher temperature values, it is brought to a temperature of 60 to 90xc2x0 C., preferably 70 to 80xc2x0 C., by cooling, which should preferably take place rapidly (15 to 30 minutes). During the stabilization phase, consisting of increasing the temperature and lowering the temperature, virtually no hydrogen is absorbed by the hydrogenated solution, and hydrogen is not significantly added to nor removed from the system, as already mentioned above.
After the hydrogenation and the thermal stabilization which is optionally carried out, the N-alkylpolyhydroxyalkylamine is obtained from the reaction product. This is preferably carried out according to the invention by first exposing the reaction product in the stirring autoclave in which the hydrogenation and, if appropriate, thermal stabilization have been carried out to a sedimentation phase while stopping the stirring. During this phase, a temperature of 60 to 90xc2x0 C., preferably 70 to 80xc2x0 C., and a hydrogen pressure of 70 to 95 bar, preferably 80 to 90 bar, is set (if this temperature and this pressure are not already present anyway) and maintained. In the sedimentation phase (which in general lasts 10 to 40 minutes), the hydrogenation catalyst is deposited on the bottom of the stirring autoclave, and an essentially clear solution is obtained, which contains the N-alkylpolyhydroxyalkylamine. The clear solution is decanted off (forced out), preferably with the aid of a dip tube introduced into the solution. During the decanting-off, the same temperature and the same hydrogen pressure as in the sedimentation are essentially maintained. The decanted-off, essentially clear solution can, if appropriate, additionally be filtered. Removal of the solvent by distillation will then be performed if it is wanted to have the N-alkylpolyhydroxyalkylamine in solvent-free form.
In the process according to the invention, the monoalkylamine and the polyol compound are employed in a molar ratio of approximately 1 to 2 to 1, preferably 1 to 1.6 to 1 (the stoichiometric molar ratio is 1 to 1). It was found that it is advantageous if monoalkylamine is also introduced into the hydrogenation autoclave in which the solution mixed under turbulence is injected for hydrogenation. The amount of alkylamine introduced can vary within wide limits and is in general 0.05 to 3 mol, preferably 0.1 to 1 mol of alkylamine per mole of polyol compound. To avoid too large a total excess of alkylamine, it is preferred to divert the whole or at least a relatively large part of the amount of alkylamine to be introduced from the superstoichiometric amount in the mentioned ratio 1 to 2 to 1 (preferably 1 to 1.6 to 1). Of this superstoichiometric amount of alkylamine, approximately 30 to 100% by weight, preferably 50 to 90% by weight, will thus be introduced into the hydrogenation system.
In the process according to the invention, the hydrogenation catalysts employed are preferably nickel catalysts, Raney nickel being particularly preferred. The amount of hydrogenation catalyst can vary within wide limits and is in general 2 to 20% by weight, preferably 5 to 15% by weight, based on the amount of polyol compound employed.
Regarding the starting compounds, monoalkylamine and reducing sugar, the following may additionally be said:
The monoalkylamine is preferably one of the formula RHN2, in which R is a preferably linear and saturated alkyl group having 1 to 18 carbon atoms, preferably having 1 to 4 carbon atoms, or a hydroxyalkyl group, preferably C1-C4-hydroxyalkyl. Examples are methylamine, ethylamine, propylamine, isopropylamine, 2-hydroxyethylamine, 2-hydroxypropylamine and the like. Methylamine or ethylamine is particularly preferred. Suitable monoalkylamines are furthermore also C1-C4-dialkylamino-C2-C6-alkylamines, for example dimethylaminopropylamine or alkoxyalkylamines of the formula ROxe2x80x94(CH2)nxe2x80x94NH2, in which R is C1-C4-alkyl and n is a number from 2 to 4.
The polyhydroxyalkyl compounds or reducing sugar compounds employed are preferably monosaccharides, preferably pentoses and hexoses, and oligosaccharides, preferably disaccharides and trisaccharides. Examples of monosaccharides are fructose, glucose, galactose, mannose, sorbose and talose as hexoses and arabinose, ribose and xylose as pentoses. Examples of oligosaccharides (polysaccharides) are lactose, maltose, maltotriose and the like. Of the oligosaccharides, the disaccharides are preferred. Particularly preferred polyols are the hexoses, in particular glucose.
The N-alkylpolyhydroxyalkylamines prepared using the process according to the invention thus correspond to the formula Rxe2x80x94NHxe2x80x94Z, in which R has the mentioned meaning and Z is a radical of the mentioned polyhydroxyalkyl compounds. Z is accordingly preferably a radical of the formula xe2x80x94CH2xe2x80x94[xe2x80x94CH(OH)]nxe2x80x94CH2OH, in which n is an integer from 3 to 5, preferably 3 or 4 and particularly preferably 4. Preferably N-C1 to C3-glucamines are thus prepared, preferably from fructose, glucose, galactose, mannose, sorbose or talose or from their mixtures and particularly preferably the corresponding glucamines such as N-methylglucamine and N-ethylglucamine.
Using the process according to the invention, a linear N-alkylpolyhydroxy-alkylamine is obtained which is very pure and is colorless in the crystalline state. It is furthermore obtained in a yield of up to 98% by weight. This is a very high yield in view of the high purity and color quality of the product. The alkylpolyhydroxy-alkylamine obtained furthermore has an only very small content of hydrogenation catalyst (nickel). It also exhibits a high thermal stability, that is it essentially retains the colorless appearance even if the product is heated to relatively high temperatures for a relatively long time. Using the process according to the invention, it is furthermore achieved that the activity of the hydrogenation catalyst is largely retained, in that, in this way, the catalyst is available for a number of batches with identical yield and product quality. As soon as the used catalyst no longer has the desired activity, it is possible to restore its original activity by addition of a relatively small amount of fresh catalyst. The process according to the invention thus guarantees a high product yield and product quality and additionally to this an only small loss of hydrogenation catalyst and catalyst activity.